The Use of Fieldbus for Process Control Applications
Process control systems and methods provide a way for ensuring efficiency, reliability, profitability, quality and safety in a process/product manufacturing environment. Such process control systems and methods can be used for automation, monitoring and control in a wide array of industrial applications for many industry segments, including textiles, glass, pulp and paper, mining, building, power, sugar, food and beverage, oil and gas, steel, water and wastewater, chemicals, etc.
The conventional process control systems and methods generally operate with a plurality of field devices positioned at various locations on, e.g., a 4–10 mA analog network. These devices include measurement and control devices (such as temperature sensors, pressure sensors, flow rate sensors, control valves, switches, etc., or combinations thereof). Recently, a number of protocols have been introduced which provide a digital alternative to conventional control systems and methods, and which utilize “smart” field devices. These “smart” field devices can provide the same functionality as the conventional devices listed above, and may additionally include one or more microprocessors, one or more memories, and other components incorporated therein. Such smart field devices can be communicatively coupled to each other and/or to a central processor using an open smart communications protocol. These protocols (e.g. FOUNDATION® Fieldbus protocol) have been widely used in manufacturing and process plants. Many of such protocols were developed for non-process control environments, such as automobile manufacturing or building automation, and were later adapted to be used for process control. Some of the more widely used fieldbus protocols include HART®, PROFIBUS®, FOUNDATION® Fieldbus, Controller Area Network protocols, etc.
Fieldbus process control systems and methods may also utilize a controller communicatively coupled to each of the smart field devices using an open, “smart” communications protocol, and a server communicatively coupled to the controller using, for example, an Ethernet connection. Moreover, this controller may include a processor, and can receive data from each of the “smart” field devices. These “smart” field devices preferably include a processor for performing certain functions thereon, without the need to use the central host for such functions. The amount of processing by the centralized host generally depends on the type of a control application and protocol used.
A smart fieldbus device, as configured by a software configurator, may be programmed to execute function blocks. A function block provides the fundamental automation functions that are performed by the process control application—function blocks are essentially a software model which defines the behavior of the process control system. More particularly, the function block is a software logic unit which processes input parameters according to a specified algorithm and an internal set of control parameters, and produces resulting output parameters that are available for use within the same function block application or by other function block applications. The input parameters of one function block may be linked to the output parameters of other function blocks on the fieldbus. The execution of each function block can be scheduled. After the function block is executed using the corresponding input values, its outputs are updated and then broadcast on the network, where they can be read by inputs of other function blocks using this information. These linked function blocks may reside either inside the same field device or in different devices on the network.
The function blocks replace many of the functions which were traditionally performed by hardware. They provide flexibility in a process control environment, since they may be modified, added or removed, without having to rewire or change the hardware of the system. Different function blocks are defined for use in FOUNDATION® fieldbus and PROFIBUS® fieldbus networks. For example, the Fieldbus Foundation establishes a set of ten standard function blocks for basic control, which are specifically defined in the FF-891 Function Blocks—Part 2 specification. This initial set of 10 function blocks released by the Fieldbus Foundation generally addresses over 80 percent of the basic process control configurations. An additional 19 standard function blocks for advanced control are defined in the FF-892 Function Blocks—Part 3 specification.
Three different types of function blocks are used in the fieldbus applications. For example, Resource Blocks define parameters that pertain to the entire application process (e.g., manufacturing ID, device type, etc.). Function Blocks encapsulate control functions (e.g., PID controller, analog input, etc.). Transducer Blocks represent an interface to sensors such as temperature, pressure and flow sensors.
Each function block in the system is identified by a unique tag which is assigned by the user. The parameters of each function block are represented by object descriptions that define how the parameters are communicated on the fieldbus network. Thus, many parameters in the system are uniquely identified by their reference to their block tag and parameter name.
Each fieldbus device likely has a Resource Block and at least one Function Block with input and/or output parameters that link to other function blocks, either in the same device or in separate devices by using the bus. Each input/output parameter includes a particular value portion and a particular status portion. The status portion of each parameter includes information regarding the reliability of the data contained in the input/output parameter, and instructs the receiving function block as to whether the reliability of contained data is acceptable, uncertain or unacceptable. In addition, a Function Block Application Process (“FBAP”) can specify the handling of control modes, alarms, events, trend reports and views. These features comply with the FOUNDATION® Fieldbus specification in order for the device to be considered interoperable at a User Layer.
Distribution of control to the field devices can be performed by synchronizing the execution of the function block and transmitting the function block parameters on the fieldbus network. Such function, along with the publication of the time of day to the devices, an automatic switch over to a redundant time publisher, an automatic assignment of device addresses, and a search for parameter names or “tags” on the fieldbus, are generally handled by System Management and Network Management.
A control strategy may be created through the interconnection of various function blocks contained by the field devices. The control strategy may also be modified without any hardware changes, thus providing another level of flexibility. The creation of the function blocks and control strategies further includes the automatic assignment of device addresses and parameter indexes. The function blocks and control strategies are described in the FOUNDATION® fieldbus and PROFIBUS® fieldbus specifications, both of which are incorporated herein by reference.
Relays
Relays are used in process control and other applications to control a load in response to a control line input as well as to control various conventional devices, such as alarm generators, limit switches and motors. Many types of relays are unintelligent devices that merely conduct a load current when an input voltage is above or below a particular threshold input value. An early conventional relay is generally an electromechanical device in which a solenoid is used to connect two switch contacts. Recently, solid state relays have become more widely used. However, these conventional relay devices are not compatible with the advanced technologies that have recently been developed for intelligent process automation and control.
Some relays may contain microprocessors and memory, and can perform logic functions: such relays are described in U.S. Pat. No. 6,360,277 the entire disclosure of which is incorporated herein by reference. The relays described in this publication are addressable, can store various protocols internally, and may therefore be inter-operable with various different process control networks. These relays can provide standard discrete outputs for the fieldbus network.
However, no fieldbus relay exists which can be easily integrated into a fieldbus control scheme by executing fieldbus function blocks, receiving power from the fieldbus network, and performing various other functions which are generally performed by FOUNDATION® or PROFIBUS® fieldbus devices. Such relay may allow the system to be homogenous, and can simplify a control strategy configuration by enabling a seamless integration of traditional discrete-controlled components into an advanced FOUNDATION® fieldbus or PROFIBUS® fieldbus process control scheme.